1. Field of the Invention
The present invention relates to optical transmission systems and optical transmitters used therein and, more specifically, to an optical transmission system in which a broadband optical signal is optically amplified for simultaneous distribution to a plurality of output ports, and an optical transmitter used in the system.
2. Description of the Background Art
An optical transmission system of a conventional type optically amplifies a broadband optical signal for simultaneous distribution to a plurality of receivers. FIG. 8 is a block diagram showing the structure of such a conventional optical transmission system. Referring to FIG. 8, the conventional optical transmission system includes an optical transmitter 1, an optical amplifier 2, a transmission path 3, a wavelength router 4, and n (where n is a natural number) pieces of optical receivers 51 to 5n.
In the optical transmission system, a broadband optical signal coming from a broadband light source and others is outputted from the optical transmitter 1 to the optical amplifier 2. The broadband optical signal is then amplified by the optical amplifier 2, and outputted to the transmission path 3. Here, the transmission path 3 is exemplified by an optical fiber and others, and transmits the optical signal amplified by the optical amplifier 2 to the wavelength router 4. The wavelength router 4 includes a plurality of output ports, and has a function of branching an incoming optical signal based on its wavelength, and from each corresponding output port provided thereto, outputs the resulting signals. Specifically, the wavelength router 4 applies wavelength branching to optical signals, and outputs the resulting optical signals from each corresponding output port. If the broadband optical signal is provided by the optical amplifier 2 via the transmission path 3, the wavelength router 4 simultaneously distributes the broadband optical signal to a plurality of output ports. The output ports of the wavelength router 4 are connected with n pieces of optical receivers 51 to 5n, respectively, in which the optical signals penetrated through the wavelength router 4 are converted into electrical signals.
However, the conventional optical transmission system as above bears the following problems. That is, if incoming power from the optical transmitter 1 increases, an outgoing power from the optical amplifier 2 saturates, resulting in gain decrease. Accordingly, in the case that the output spectrum from the optical transmitter 1 is spreading over a wide wavelength bandwidth as above, the incoming power in the optical amplifier 2 becomes increased, so that gain acquisition in the optical amplifier 2 cannot be effectively done. Further, if the conventional optical system applies a technique of multiplexing a broadband optical signal with any other optical signal, the resulting output spectrum from the optical transmitter 1 spreads over the wide wavelength bandwidth. As a result, the output spectrum affects other multiplexing optical signals as does cover their wavelength bandwidths as well.